EP0114008A2 - Telephone set with receiving amplifier provided with an anti-Larsen arrangement - Google Patents

Abstract

1. Amplified speaker telephone apparatus comprising a microphone, a signal subtractor (16) receiving a signal (M) from the microphone and a signal (S) from a telephone line, an amplifier (22) and a loudspeaker for amplified listening (24) receiving the signal from the subtractor, and an ""antilarsen" loop (26) receiving the signal from the microphone and acting on the listening amplifier for reducing the amplitude of the signal applied to the loudspeaker when the signal from the microphone exceeds a certain level, characterized in that the antilarsen loop includes a high-pass or band-pass filter (34) having a high attenuation for the frequencies below about 1 kilohertz and a low attenuation for frequencies where the larsen effect most likely occurs.

Description

There is an increasing tendency to provide amplified listening loudspeakers on subscriber telephone sets allowing the voice of a correspondent to be heard clearly, without the use of additional headphones, by persons who are at a few meters from the post.

These stations include a microphone-earpiece handset, used in any case by a single person who may or may not activate the amplified listening system or a fixed microphone in "hands-free" telephones.

The powered listening speaker can be located on the station itself; it is then fixed, while the handset is connected to the station by a wire and is mobile. In addition, the loudspeaker is often turned upwards, that is to say directly towards the person holding the handset.

As a result, there can sometimes be a "feedback" effect, that is to say a loud hissing sound resulting from the fact that the microphone of the handset directly receives a sound from the loudspeaker and sends it back into the circuit where it again amplified by the speaker. This phenomenon of resonance is well known and is very unpleasant to the ear. It can occur on an amplified listening telephone set as long as the handset microphone is too close to the speaker.

We therefore sought to remedy this defect to obtain better listening comfort. The generally adopted system consists in providing in the amplified listening system an "antilarsen" loop which receives the signal coming from the microphone of the handset and which acts so as to reduce the amplitude of the signal applied to the loudspeaker of amplified listening if the average level of signal received by the microphone is too high, as a too high level may indicate that a feedback effect is being established.

Such a system is represented in FIG. 1. A telephone signal S (signal transmission and reception mixed) is present to an input A connected to a telephone line, and a signal M arrives at an input B connected to a microphone of the handset (not shown). The signal M is subtracted, after amplification in a preamplifier 10, from the signal S present on the line, in a subtractor 16 also called "antilocal system" because it prevents the passage to the earphones of the signal emitted locally, namely the voice of the person holding the telephone handset. The output signal from subtractor 16 now only includes the reception signal, that is to say the voice of the correspondent at the other end of the line.

The output of the subtractor is applied to a first amplifier 18, at the output of which the earphone 20 of the handset is connected. The signal thus amplified is applied to a second amplifier 22, with variable gain, which supplies a signal to a loudspeaker 24.

An antilarsen loop, represented in a dotted frame 26, controls the gain of the amplifier 22 as a function of the signal M received from the microphone. The loop includes a rectifier 28, a smoothing capacity 30, integrating the rectified signal with a time constant sufficient to provide an indication of the average level of the signal M, and an amplifier or a comparator with threshold 32 which controls the gain of the amplifier 22 to make it fall if the signal from the microphone 10 exceeds a given threshold, and all the more so when this threshold is exceeded.

This system has been tried and marketed but it is not satisfactory because, despite its simplicity, its adjustment turns out to be very delicate in the end. Indeed, it has appeared that the anti-feedback loop is only sufficiently effective if the comparator 32 causes the gain to drop sharply when the microphone output level is such that a feedback phenomenon is likely to occur. But the sound level of the feedback effect is not necessarily very well known and we are led in practice to set the comparator to a very low level. But then, it turns out that the amplified listening is cut off as soon as the microphone records a noise, and in particular when the user who holds the handset speaks weakly or even breathes a little loud in front of his microphone.

A compromise is therefore difficult to find if it is necessary on the one hand to eliminate the feedback effect even when the microphone is very close to the loudspeaker, and on the other hand to avoid an untimely interruption or a reduction in the amplified listening in lack of feedback.

• - la première est de disposer le haut parleur d'écoute amplifiée de telle sorte qu'il soit difficile à l'utilisateur de rapprocher le microphone du combiné suffisamment près pour déclencher un effet larsen ; mais la place disponible sur un poste téléphonique n'est pas grande et le combiné est relié au poste par un fil de sorte qu'il est toujours possible à l'utilisateur de le déplacer n'importe où. De plus, si on tourne le haut-parleur vers une face arrière, a priori peu accessible du poste, l'écoute amplifiée sera moins bonne pour les auditeurs ;
• - la deuxième solution consisterait à perfectionner le soustracteur 16. En effet, s'il était parfait, ce soustracteur devrait empêcher tout effet larsen puisque le signal issu du micro n'est en principe pas transmis au haut-parleur d'écoute. En fait, ce système ne peut être parfait et le perfectionner aboutirait à un coût élevé qu'on ne souhaite pas. D'autre part, il est très désagréable de ne pas s'entendre du tout parler dans l'écouteur : on a l'impression que le téléphone ne fonctionne pas.

The solutions to reach a valid compromise consist a priori in two possible ways:

• - The first is to arrange the amplified listening speaker so that it is difficult for the user to bring the microphone to the handset close enough to trigger a feedback effect; but the space available on a telephone set is not large and the handset is connected to the set by a wire so that it is always possible for the user to move it anywhere. In addition, if we turn the loudspeaker towards a rear face, a priori hardly accessible from the station, the amplified listening will be less good for the listeners;
• - The second solution would be to perfect the subtractor 16. Indeed, if it were perfect, this subtractor should prevent any feedback effect since the signal from the microphone is in principle not transmitted to the listening speaker. In fact, this system cannot be perfect and perfecting it would result in an unwanted high cost. On the other hand, it is very unpleasant not to hear yourself speak at all in the earpiece: one has the impression that the phone does not work.

To very effectively improve the functioning of an antilarsen loop without redesigning the design of the telephone set, without rejecting the amplified listening speaker at an inconvenient place, and without looking for complex and expensive solutions to the improvement of the antilocal system, the present application proposes a particularly simple solution based on a detailed analysis of the problems mentioned above.

This analysis showed that the feedback effect always occurs at a relatively high frequency in the bandwidth of the microphone, this mainly because of the frequency response curve of the latter which presents its maximum at neighborhood of 2 to 3 kilohertz.

On the contrary, the noises which unnecessarily cut off the amplified listening (noise of speech of the user, noises of the surroundings, noisy breathing, etc.) have the maximum energy in a frequency band below 1 kHz.

There is provided according to the invention a solution taking into account these circumstances, which consists in inserting into the antilarsen loop, upstream of the rectifier, a high-pass or band-pass filter having a strong attenuation for frequencies below about 1 kilohertz and a weak attenuation for a frequency range in which the feedback effect is most likely to occur. The difference between the strong attenuation (at around 1 kilohertz and below) and the weak attenuation is preferably at least 6 decibels.

In a first embodiment, the filter has low attenuation for frequencies between 2 and 3 kilohertz approximately.

In another embodiment, artificially shifting at a high frequency, outside the audible bandwidth (300 - 3400 Hz), the frequency at which the feedback effect will be established, and it is expected that the filter has a strong attenuation throughout the audible bandwidth and a low attenuation in the frequency range, outside this bandwidth, for which the feedback effect is likely to occur. To impose the frequency of the feedback effect, an amplifier with a very selective frequency response curve, flat in the bandwidth of the microphone and having a high peak at a high frequency, is inserted upstream of the amplified listening loudspeaker. outside this bandwidth (for example at 10 kilohertz).

• - la figure 1 déjà décrite représente schématiquement un poste téléphonique à écoute amplifiée de type connu ;
• - la figure 2 représente le poste perfectionné selon l'invention ;
• - la figure 3 représente un exemple de bande passante de filtre inséré dans la boucle antilarsen, avec en pointillés une courbe de réponse en fréquence typique de microphone ;
• - les figures 4 et 5 représentent des exemples simples de filtres passe-bande et passe-haut pouvant être insérés dans la boucle antilarsen ;
• - la figure 6 représente un autre mode de réalisation de l'invention, incorporant un amplificateur sélectif ;
• - la figure 7 représente la courbe de réponse de l'amplificateur sélectif de la figure 6 ;
• - la figure 8 représente une courbe de réponse combinant la réponse du microphone du combiné et de l'amplificateur sélectif de la figure 6.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows and which is given with reference to the appended drawings in which:

• - Figure 1 already described schematically shows an amplified listening telephone of known type;
• - Figure 2 shows the improved position according to the invention;
• - Figure 3 shows an example of filter bandwidth inserted in the antilarsen loop, with dotted lines a typical frequency response curve of microphone;
• - Figures 4 and 5 show simple examples of bandpass and highpass filters that can be inserted into the antilarsen loop;
• - Figure 6 shows another embodiment of the invention, incorporating a selective amplifier;
• - Figure 7 shows the response curve of the selective amplifier of Figure 6;
• FIG. 8 represents a response curve combining the response of the microphone of the handset and the selective amplifier of FIG. 6.

Compared to FIG. 1, the circuit according to the invention is modified by the insertion, between the amplifier 10 receiving the signal from the microphone and the rectifier 28, a high-pass or band-pass filter 34.

It will be understood that the choice of a band pass filter rather than a high pass filter is dictated by secondary considerations such as the elimination of noise, parasites, etc., but that the essential characteristic of the filter is its aptitude. to strongly attenuate the frequencies below approximately 1000 hertz and to let pass with a much weaker attenuation the frequencies in the frequency range where the feedback effect is most likely to occur. So we are not concerned here with knowing whether the filter should cut very high frequencies (for example beyond 10 kilohertz).

In the example in Figure 2, the feedback effect occurs at a frequency between 2 and 3 kilohertz because it is in this range that the microphone has a stronger response.

In FIG. 3, there has been shown in dotted lines a frequency response curve typical of the cascade assembly of a telephone handset microphone, the amplifier 10, the antilocal system 16, the amplifiers 18 and 22 and the loudspeaker 24. The shape of this curve is essentially imposed by the microphone. We have represented in solid lines an example of a possible response curve for filter 34. The scale of the amplitudes on the ordinate (Attenuation At) matters little and was not indicated voluntarily.

The attenuation around 1000 hertz is at least about 6 decibels compared to the maximum attenuation of the response curve of the filter 34.

The filter used can be very simple, for example a well-known Sallen and Key cell, bandpass or highpass, two examples of which are shown in Figures 4 and 5 respectively. These filters are second order filters with attenuation 12 dB per octave below their low cutoff frequency.

If the beginning of a feedback effect occurs, it will be for a frequency located towards the top of the curve in dotted lines in FIG. 3. The comparator 32 will detect the exceeding of a threshold of sound level recorded by the microphone and will decrease the gain of the amplifier 22 upstream of the amplified listening loudspeaker 24, this all the more as the signal received by the microphone will be greater.

But if a noise level, even important, is recorded by the microphone in a frequency range below about 1000 hertz (noise of words, ambient noise, strong breathing), this noise attenuated by the filter 34 will not be enough to toggle the comparator 32. It will therefore not, rightly, be considered as a start of feedback effect justifying the cut-off of the amplified listening loudspeaker.

• Tout d'abord, on a inséré en amont de l'amplificateur 22 un amplificateur auxiliaire sélectif 36 qui a une courbe de réponse en fréquence très particulière en ce sens qu'elle est sensiblement plate dans la bande de fréquences audibles utile (par exemple de 300 hertz à 3,4 kilohertz pour un appareil téléphonique), et qu'elle présente un pic élevé dans une bande de fréquences plus élevée, en dehors de la bande passante du microphone. Par exemple, ce pic se situe entre 5 et 10 kilohertz. La courbe de réponse correspondante est représentée à la figure 7.

Another embodiment of the invention is shown in FIG. 6. This embodiment differs from that which has been described above on three points:

• First of all, a selective auxiliary amplifier 36 has been inserted upstream of the amplifier 22 which has a very specific frequency response curve in the sense that it is substantially flat in the useful audible frequency band (for example of 300 hertz at 3.4 kilohertz for a telephone device), and that it presents a high peak in a higher frequency band, outside the bandwidth of the microphone. For example, this peak is between 5 and 10 kilohertz. The corresponding response curve is shown in Figure 7.

The chain comprising the microphone, the amplifier 10, the antilocal system 16, the amplifier 18, the amplifier 36, the amplifier 22 and the loudspeaker 24 then has a frequency response curve which is that of the figure 8. The amplitude of the peak is chosen such that the curve of figure 8 itself presents an absolute maximum for a frequency corresponding to this peak (between 5 and 10 kilohertz): it is then at this frequency that a feedback effect is likely to occur.

Consequently, the second modification made to the circuit of FIG. 3 is the fact that the antilarsen loop includes a high-pass filter (or band-pass) whose low cut-off frequency is not of the order of 1 kilohertz but rather on the order of 3 to 5 kilohertz.

The third modification is the insertion of a high gain amplifier 38 upstream or downstream of the high pass filter 34, allowing a frequency resulting from the feedback effect (for example at 7 kilohertz) to be sufficiently amplified to compensate the very weak response of the microphone at this frequency and to trigger the weakening of the amplified listening.

In this way, no noise from the microphone, not even noise at around 3 kilohertz, will trigger a weakening of the amplified listening, because this noise will be neutralized by the high-pass filter. However, noise over 5 kilohertz will be detected by comparator 32 and will lead to a weakening of the amplified listening. It should also be noted that the rejection of the feedback effect at a frequency of 10 kilohertz is interesting because it is much less annoying from a physiological point of view than a feedback effect at 3 kilohertz.

Finally, it can be indicated that the selective amplifier 36 and the amplifier 18 can be combined in a single amplifier or in practice in two amplifiers in parallel, one of which is very selective and only amplifies the frequencies around 10 kilohertz and l the other has a flat frequency response curve up to at least 10 to 15 kilohertz.

Claims (5)

1. Amplified listening telephone apparatus comprising a microphone, a signal subtractor (16) receiving a signal (M) from the microphone and a signal (S) from a telephone line, an amplifier (22) and a speaker amplified listening device (24) receiving the signal from the subtractor, and an "antilarsen" loop (26) receiving the signal coming from the microphone and acting on the amplified listening amplifier to reduce the amplitude of the signal applied to the loudspeaker amplified listening speaker if the signal from the microphone exceeds a certain level, characterized in that the antilarsen loop includes a high-pass or band-pass filter (34) having a strong attenuation for frequencies below about 1 kilohertz and low attenuation for frequencies at which a feedback effect is most likely to occur. 2. Telephone apparatus according to claim 1, characterized in that the antilarsen loop comprises the high-pass or band-pass filter (34) followed by a rectifier (28), a smoothing capacity (30), and d 'a threshold comparator (32). 3. Telephone apparatus according to claim 1, characterized in that the high-pass or band-pass filter has a low attenuation for frequencies of about 2 to 3 kilohertz. 4. Telephone apparatus according to claim 1, characterized in that there is provided, between the subtractor and the amplified listening loudspeaker, a selective amplifier (36) having a frequency response curve having, in the passband of the microphone, a substantially flat response and, at a higher frequency a peak of amplitude sufficiently high for the feedback effect to occur preferentially at a frequency corresponding to this peak. 5. Telephone apparatus according to claim 4, characterized in that the high-pass or band-pass filter has a high attenuation throughout the pass band of the microphone.

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